A. Field of the Invention
The invention relates to a method for increasing the permeability of a subterranean formation, wherein the permeability increase is achieved by contacting the formation with an acidic solution to dissolve a portion of the formation. It more particularly relates to an acidizing method of the type employing mud acid.
B. Description of the Prior Art
Numerous procedures for treating wells with siliceous-material-dissolving acids are known. A good discussion of the known art is found in columns 1 and 2 of Templeton et al., U.S. Pat. No. 3,828,854 and in the "Introduction" section of Society of Petroleum Engineers Paper No. 5153, which paper relates to the same invention as the Templeton et al. patent.
Conventionally, siliceous formations have been acidized by contact with mud acid. As used herein, "mud acid" refers to an aqueous solution of hydrofluoric acid and at least one of hydrochloric acid, acetic acid, or formic acid; usually, the acid in addition to HF is HCl. As is well understood in the art, the derivation of the HCl and HF is not critical, so that "mud acid" also includes aqueous solutions of chemicals which quickly react to form HCl and HF, i.e., so that by the time the solution reaches the formation, the active ingredients are HF and HCl. The respective concentrations of HCl and HF may vary over wide ranges, with the lower limits being more a matter of practicality rather than operability, and the upper limits being a matter of mutual solubility of the two acids. Thus, any given mud acid solution may have an HCl concentration, by weight, of from about 1 percent or even less up to about 37 percent, and an HF concentration of from about 0.5 percent or even less up to about 25 percent, though as the upper limit is approached for one species, a lesser concentration of the other may be required because of solubility limitations. Most typically, a mud acid is substantially free of other acidic species, consisting substantially of from about 3 to about 25 percent HCl and about 1 to about 10 HF. A mud acid may also contain one or more functional additives such as inhibitors, diverting agents, and/or surfactants, and such functional additives are not to be considered excluded by the expression "consisting substantially of".
Conventional treatments of siliceous clay containing formations with mud acids have generally given excellent results for a short time, but the improvements in production are frequently short lived, with a rapid decline in production being observed thereafter. It has been hypothesized that this phenomenon is observed because the mud acid reacts rapidly with the formation in the first few inches around the borehole, thus spending so rapidly that penetration deep into the formation is not achieved. Subsequently, fines in the surrounding formation migrate into the vicinity of the borehole and replug the acidized portion of the formation.
One approach to this problem is that taught by Templeton et al. in the aforementioned patent and publication. They teach to inject a composition which generates HF slowly, and thus enables the solution to be placed in contact with the formation before any significant amount of the HF is generated. The system there described is a relatively high pH (.gtoreq.2) aqueous solution of a water soluble fluoride salt and at least one water reactive organic acid ester. From the examples in the patent and paper, it appears that the ester most preferred by Templeton et al. is methyl formate.
An alternative method for acidizing sand formations would be desirable, however, since the method of Templeton et al. suffers from at least two drawbacks. First, many of the organic esters are highly flammable materials which objectionable from a safety standpoint. Second, as Templeton et al. acknowledge, the fluoride salt-organic ester system actually causes at least temporary formation damage since it causes precipitation of biproducts such as ralstonite.
As further background, the use of fluoboric acid in well treating has been previously described. Ayers, Jr., U.S. Pat. No. 2,300,393 teaches treatment with fluoboric acid, optionally with small amounts of HF. Ayers, Jr., warns against using large excesses of HF. Ayers, Jr., also teaches the fluoboric acid may be followed by HCl containing no appreciable amount of hydrofluoric acid, or optionally, by a mixture of HCl and fluoboric acids. Bond et al., U.S. Pat. No. 2,425,415 teaches an acidizing procedure wherein the formation is first contacted with a fluoboric acid solution which contains no free HF, but which contains an excess of boric acid, and thereafter with aqueous fluoboric acid containing excess HF. Kingston et al., U.S. Pat. No. 2,663,689 describes the use of boric acid in aqueous HCl-HF to avoid precipitation of insoluble fluoride salts and fluorosilicic acid.